1 /* @(#)e_pow.c 1.5 04/04/22 SMI */
2 /*
3  * ====================================================
4  * Copyright (C) 2004 by Sun Microsystems, Inc. All rights reserved.
5  *
6  * Permission to use, copy, modify, and distribute this
7  * software is freely granted, provided that this notice
8  * is preserved.
9  * ====================================================
10  */
11 
12 #include <sys/cdefs.h>
13 __FBSDID("$FreeBSD$");
14 
15 /* __ieee754_pow(x,y) return x**y
16  *
17  *		      n
18  * Method:  Let x =  2   * (1+f)
19  *	1. Compute and return log2(x) in two pieces:
20  *		log2(x) = w1 + w2,
21  *	   where w1 has 53-24 = 29 bit trailing zeros.
22  *	2. Perform y*log2(x) = n+y' by simulating multi-precision
23  *	   arithmetic, where |y'|<=0.5.
24  *	3. Return x**y = 2**n*exp(y'*log2)
25  *
26  * Special cases:
27  *	1.  (anything) ** 0  is 1
28  *	2.  (anything) ** 1  is itself
29  *	3.  (anything) ** NAN is NAN except 1 ** NAN = 1
30  *	4.  NAN ** (anything except 0) is NAN
31  *	5.  +-(|x| > 1) **  +INF is +INF
32  *	6.  +-(|x| > 1) **  -INF is +0
33  *	7.  +-(|x| < 1) **  +INF is +0
34  *	8.  +-(|x| < 1) **  -INF is +INF
35  *	9.  +-1         ** +-INF is 1
36  *	10. +0 ** (+anything except 0, NAN)               is +0
37  *	11. -0 ** (+anything except 0, NAN, odd integer)  is +0
38  *	12. +0 ** (-anything except 0, NAN)               is +INF
39  *	13. -0 ** (-anything except 0, NAN, odd integer)  is +INF
40  *	14. -0 ** (odd integer) = -( +0 ** (odd integer) )
41  *	15. +INF ** (+anything except 0,NAN) is +INF
42  *	16. +INF ** (-anything except 0,NAN) is +0
43  *	17. -INF ** (anything)  = -0 ** (-anything)
44  *	18. (-anything) ** (integer) is (-1)**(integer)*(+anything**integer)
45  *	19. (-anything except 0 and inf) ** (non-integer) is NAN
46  *
47  * Accuracy:
48  *	pow(x,y) returns x**y nearly rounded. In particular
49  *			pow(integer,integer)
50  *	always returns the correct integer provided it is
51  *	representable.
52  *
53  * Constants :
54  * The hexadecimal values are the intended ones for the following
55  * constants. The decimal values may be used, provided that the
56  * compiler will convert from decimal to binary accurately enough
57  * to produce the hexadecimal values shown.
58  */
59 
60 #include "math.h"
61 #include "math_private.h"
62 
63 static const double
64 bp[] = {1.0, 1.5,},
65 dp_h[] = { 0.0, 5.84962487220764160156e-01,}, /* 0x3FE2B803, 0x40000000 */
66 dp_l[] = { 0.0, 1.35003920212974897128e-08,}, /* 0x3E4CFDEB, 0x43CFD006 */
67 zero    =  0.0,
68 one	=  1.0,
69 two	=  2.0,
70 two53	=  9007199254740992.0,	/* 0x43400000, 0x00000000 */
71 huge	=  1.0e300,
72 tiny    =  1.0e-300,
73 	/* poly coefs for (3/2)*(log(x)-2s-2/3*s**3 */
74 L1  =  5.99999999999994648725e-01, /* 0x3FE33333, 0x33333303 */
75 L2  =  4.28571428578550184252e-01, /* 0x3FDB6DB6, 0xDB6FABFF */
76 L3  =  3.33333329818377432918e-01, /* 0x3FD55555, 0x518F264D */
77 L4  =  2.72728123808534006489e-01, /* 0x3FD17460, 0xA91D4101 */
78 L5  =  2.30660745775561754067e-01, /* 0x3FCD864A, 0x93C9DB65 */
79 L6  =  2.06975017800338417784e-01, /* 0x3FCA7E28, 0x4A454EEF */
80 P1   =  1.66666666666666019037e-01, /* 0x3FC55555, 0x5555553E */
81 P2   = -2.77777777770155933842e-03, /* 0xBF66C16C, 0x16BEBD93 */
82 P3   =  6.61375632143793436117e-05, /* 0x3F11566A, 0xAF25DE2C */
83 P4   = -1.65339022054652515390e-06, /* 0xBEBBBD41, 0xC5D26BF1 */
84 P5   =  4.13813679705723846039e-08, /* 0x3E663769, 0x72BEA4D0 */
85 lg2  =  6.93147180559945286227e-01, /* 0x3FE62E42, 0xFEFA39EF */
86 lg2_h  =  6.93147182464599609375e-01, /* 0x3FE62E43, 0x00000000 */
87 lg2_l  = -1.90465429995776804525e-09, /* 0xBE205C61, 0x0CA86C39 */
88 ovt =  8.0085662595372944372e-0017, /* -(1024-log2(ovfl+.5ulp)) */
89 cp    =  9.61796693925975554329e-01, /* 0x3FEEC709, 0xDC3A03FD =2/(3ln2) */
90 cp_h  =  9.61796700954437255859e-01, /* 0x3FEEC709, 0xE0000000 =(float)cp */
91 cp_l  = -7.02846165095275826516e-09, /* 0xBE3E2FE0, 0x145B01F5 =tail of cp_h*/
92 ivln2    =  1.44269504088896338700e+00, /* 0x3FF71547, 0x652B82FE =1/ln2 */
93 ivln2_h  =  1.44269502162933349609e+00, /* 0x3FF71547, 0x60000000 =24b 1/ln2*/
94 ivln2_l  =  1.92596299112661746887e-08; /* 0x3E54AE0B, 0xF85DDF44 =1/ln2 tail*/
95 
96 double
__ieee754_pow(double x,double y)97 __ieee754_pow(double x, double y)
98 {
99 	double z,ax,z_h,z_l,p_h,p_l;
100 	double y1,t1,t2,r,s,t,u,v,w;
101 	int32_t i,j,k,yisint,n;
102 	int32_t hx,hy,ix,iy;
103 	u_int32_t lx,ly;
104 
105 	EXTRACT_WORDS(hx,lx,x);
106 	EXTRACT_WORDS(hy,ly,y);
107 	ix = hx&0x7fffffff;  iy = hy&0x7fffffff;
108 
109     /* y==zero: x**0 = 1 */
110 	if((iy|ly)==0) return one;
111 
112     /* x==1: 1**y = 1, even if y is NaN */
113 	if (hx==0x3ff00000 && lx == 0) return one;
114 
115     /* y!=zero: result is NaN if either arg is NaN */
116 	if(ix > 0x7ff00000 || ((ix==0x7ff00000)&&(lx!=0)) ||
117 	   iy > 0x7ff00000 || ((iy==0x7ff00000)&&(ly!=0)))
118 		return (x+0.0)+(y+0.0);
119 
120     /* determine if y is an odd int when x < 0
121      * yisint = 0	... y is not an integer
122      * yisint = 1	... y is an odd int
123      * yisint = 2	... y is an even int
124      */
125 	yisint  = 0;
126 	if(hx<0) {
127 	    if(iy>=0x43400000) yisint = 2; /* even integer y */
128 	    else if(iy>=0x3ff00000) {
129 		k = (iy>>20)-0x3ff;	   /* exponent */
130 		if(k>20) {
131 		    j = ly>>(52-k);
132 		    if((j<<(52-k))==ly) yisint = 2-(j&1);
133 		} else if(ly==0) {
134 		    j = iy>>(20-k);
135 		    if((j<<(20-k))==iy) yisint = 2-(j&1);
136 		}
137 	    }
138 	}
139 
140     /* special value of y */
141 	if(ly==0) {
142 	    if (iy==0x7ff00000) {	/* y is +-inf */
143 	        if(((ix-0x3ff00000)|lx)==0)
144 		    return  one;	/* (-1)**+-inf is 1 */
145 	        else if (ix >= 0x3ff00000)/* (|x|>1)**+-inf = inf,0 */
146 		    return (hy>=0)? y: zero;
147 	        else			/* (|x|<1)**-,+inf = inf,0 */
148 		    return (hy<0)?-y: zero;
149 	    }
150 	    if(iy==0x3ff00000) {	/* y is  +-1 */
151 		if(hy<0) return one/x; else return x;
152 	    }
153 	    if(hy==0x40000000) return x*x; /* y is  2 */
154 	    if(hy==0x3fe00000) {	/* y is  0.5 */
155 		if(hx>=0)	/* x >= +0 */
156 		return sqrt(x);
157 	    }
158 	}
159 
160 	ax   = fabs(x);
161     /* special value of x */
162 	if(lx==0) {
163 	    if(ix==0x7ff00000||ix==0||ix==0x3ff00000){
164 		z = ax;			/*x is +-0,+-inf,+-1*/
165 		if(hy<0) z = one/z;	/* z = (1/|x|) */
166 		if(hx<0) {
167 		    if(((ix-0x3ff00000)|yisint)==0) {
168 			z = (z-z)/(z-z); /* (-1)**non-int is NaN */
169 		    } else if(yisint==1)
170 			z = -z;		/* (x<0)**odd = -(|x|**odd) */
171 		}
172 		return z;
173 	    }
174 	}
175 
176     /* CYGNUS LOCAL + fdlibm-5.3 fix: This used to be
177 	n = (hx>>31)+1;
178        but ANSI C says a right shift of a signed negative quantity is
179        implementation defined.  */
180 	n = ((u_int32_t)hx>>31)-1;
181 
182     /* (x<0)**(non-int) is NaN */
183 	if((n|yisint)==0) return (x-x)/(x-x);
184 
185 	s = one; /* s (sign of result -ve**odd) = -1 else = 1 */
186 	if((n|(yisint-1))==0) s = -one;/* (-ve)**(odd int) */
187 
188     /* |y| is huge */
189 	if(iy>0x41e00000) { /* if |y| > 2**31 */
190 	    if(iy>0x43f00000){	/* if |y| > 2**64, must o/uflow */
191 		if(ix<=0x3fefffff) return (hy<0)? huge*huge:tiny*tiny;
192 		if(ix>=0x3ff00000) return (hy>0)? huge*huge:tiny*tiny;
193 	    }
194 	/* over/underflow if x is not close to one */
195 	    if(ix<0x3fefffff) return (hy<0)? s*huge*huge:s*tiny*tiny;
196 	    if(ix>0x3ff00000) return (hy>0)? s*huge*huge:s*tiny*tiny;
197 	/* now |1-x| is tiny <= 2**-20, suffice to compute
198 	   log(x) by x-x^2/2+x^3/3-x^4/4 */
199 	    t = ax-one;		/* t has 20 trailing zeros */
200 	    w = (t*t)*(0.5-t*(0.3333333333333333333333-t*0.25));
201 	    u = ivln2_h*t;	/* ivln2_h has 21 sig. bits */
202 	    v = t*ivln2_l-w*ivln2;
203 	    t1 = u+v;
204 	    SET_LOW_WORD(t1,0);
205 	    t2 = v-(t1-u);
206 	} else {
207 	    double ss,s2,s_h,s_l,t_h,t_l;
208 	    n = 0;
209 	/* take care subnormal number */
210 	    if(ix<0x00100000)
211 		{ax *= two53; n -= 53; GET_HIGH_WORD(ix,ax); }
212 	    n  += ((ix)>>20)-0x3ff;
213 	    j  = ix&0x000fffff;
214 	/* determine interval */
215 	    ix = j|0x3ff00000;		/* normalize ix */
216 	    if(j<=0x3988E) k=0;		/* |x|<sqrt(3/2) */
217 	    else if(j<0xBB67A) k=1;	/* |x|<sqrt(3)   */
218 	    else {k=0;n+=1;ix -= 0x00100000;}
219 	    SET_HIGH_WORD(ax,ix);
220 
221 	/* compute ss = s_h+s_l = (x-1)/(x+1) or (x-1.5)/(x+1.5) */
222 	    u = ax-bp[k];		/* bp[0]=1.0, bp[1]=1.5 */
223 	    v = one/(ax+bp[k]);
224 	    ss = u*v;
225 	    s_h = ss;
226 	    SET_LOW_WORD(s_h,0);
227 	/* t_h=ax+bp[k] High */
228 	    t_h = zero;
229 	    SET_HIGH_WORD(t_h,((ix>>1)|0x20000000)+0x00080000+(k<<18));
230 	    t_l = ax - (t_h-bp[k]);
231 	    s_l = v*((u-s_h*t_h)-s_h*t_l);
232 	/* compute log(ax) */
233 	    s2 = ss*ss;
234 	    r = s2*s2*(L1+s2*(L2+s2*(L3+s2*(L4+s2*(L5+s2*L6)))));
235 	    r += s_l*(s_h+ss);
236 	    s2  = s_h*s_h;
237 	    t_h = 3.0+s2+r;
238 	    SET_LOW_WORD(t_h,0);
239 	    t_l = r-((t_h-3.0)-s2);
240 	/* u+v = ss*(1+...) */
241 	    u = s_h*t_h;
242 	    v = s_l*t_h+t_l*ss;
243 	/* 2/(3log2)*(ss+...) */
244 	    p_h = u+v;
245 	    SET_LOW_WORD(p_h,0);
246 	    p_l = v-(p_h-u);
247 	    z_h = cp_h*p_h;		/* cp_h+cp_l = 2/(3*log2) */
248 	    z_l = cp_l*p_h+p_l*cp+dp_l[k];
249 	/* log2(ax) = (ss+..)*2/(3*log2) = n + dp_h + z_h + z_l */
250 	    t = (double)n;
251 	    t1 = (((z_h+z_l)+dp_h[k])+t);
252 	    SET_LOW_WORD(t1,0);
253 	    t2 = z_l-(((t1-t)-dp_h[k])-z_h);
254 	}
255 
256     /* split up y into y1+y2 and compute (y1+y2)*(t1+t2) */
257 	y1  = y;
258 	SET_LOW_WORD(y1,0);
259 	p_l = (y-y1)*t1+y*t2;
260 	p_h = y1*t1;
261 	z = p_l+p_h;
262 	EXTRACT_WORDS(j,i,z);
263 	if (j>=0x40900000) {				/* z >= 1024 */
264 	    if(((j-0x40900000)|i)!=0)			/* if z > 1024 */
265 		return s*huge*huge;			/* overflow */
266 	    else {
267 		if(p_l+ovt>z-p_h) return s*huge*huge;	/* overflow */
268 	    }
269 	} else if((j&0x7fffffff)>=0x4090cc00 ) {	/* z <= -1075 */
270 	    if(((j-0xc090cc00)|i)!=0) 		/* z < -1075 */
271 		return s*tiny*tiny;		/* underflow */
272 	    else {
273 		if(p_l<=z-p_h) return s*tiny*tiny;	/* underflow */
274 	    }
275 	}
276     /*
277      * compute 2**(p_h+p_l)
278      */
279 	i = j&0x7fffffff;
280 	k = (i>>20)-0x3ff;
281 	n = 0;
282 	if(i>0x3fe00000) {		/* if |z| > 0.5, set n = [z+0.5] */
283 	    n = j+(0x00100000>>(k+1));
284 	    k = ((n&0x7fffffff)>>20)-0x3ff;	/* new k for n */
285 	    t = zero;
286 	    SET_HIGH_WORD(t,n&~(0x000fffff>>k));
287 	    n = ((n&0x000fffff)|0x00100000)>>(20-k);
288 	    if(j<0) n = -n;
289 	    p_h -= t;
290 	}
291 	t = p_l+p_h;
292 	SET_LOW_WORD(t,0);
293 	u = t*lg2_h;
294 	v = (p_l-(t-p_h))*lg2+t*lg2_l;
295 	z = u+v;
296 	w = v-(z-u);
297 	t  = z*z;
298 	t1  = z - t*(P1+t*(P2+t*(P3+t*(P4+t*P5))));
299 	r  = (z*t1)/(t1-two)-(w+z*w);
300 	z  = one-(r-z);
301 	GET_HIGH_WORD(j,z);
302 	j += (n<<20);
303 	if((j>>20)<=0) z = scalbn(z,n);	/* subnormal output */
304 	else SET_HIGH_WORD(z,j);
305 	return s*z;
306 }
307